Milbemycin is natural product from microbial source, which can be used as pesticide, and there have been data showing that it is one of the best acaricides in the modern world. The US Environmental Protection Agency identifies it as a low-risk pesticide and Holland approves it as “GNO” (natural product in the production of crops). It belongs to ecology friendly pesticide and is applicable to the comprehensive prevention and control of organic agriculture pests and diseases, which has become a popular insecticidal and acaricidal agent in developed countries.
Milbemycin is a metabolite having insecticidal activity, which was screened from the fermentation broth of microorganism by Sankyo Company Limited of Japan using two-spotted spider mites as test insects (U.S. Pat. No. 3,950,360). After extensive fundamental researches, the mixture of the components of milbemycin A3 and milbemycin A4 (A3:A4=3:7) was used as an acaricide in 1983. The structures of milbemycin A3 and milbemycin A4 are as shown in formula I. In 1990, 1% of milbemycin emulsifiable concentrate (milbeknock) was used as an acaricide for tea and eggplants in Japan. In 1993, 1% of milbemycin emulsifiable concentrate was also registered in Japan as a pesticide for pears, peaches, watermelons, strawberries, eggplants and flowers. Currently, milbemycin has been registered in many countries, such as Japan and many countries in Europe and America, etc. and is recommended by the US Environmental Protection Agency to be used as a safe, environment friendly insecticidal and acaricidal agent.
Formula I. The structures of milbemycin A3 and milbemycin A4.
Sankyo Company Limited of Japan is the original factory of milbemycin and its literature published in 1980 showed that the content of milbemycin A3+ milbemycin A4 was only about 150 mg/L (Yo Takiguchi et al, The J. of Antibiotics, 1980, October, 1120-1127). After mutagenesis of strain and optimization of fermentation conditions, in 1990, the content of milbemycin A3+ milbemycin A4 reached 530 mg/L (Koicchi Nonaka et al, Actinomycetol. 1990, 13:32-41). The domestic research of milbemycin started later, mainly concentrated in XIANG Wensheng research group of Northeast Agricultural University, and has also made a great breakthrough currently. For example, the unit content of milbemycin A3+ milbemycin A4 reached a maximum of 1595 mg/L in the literature published in 2011 (Zhang Baoxin et al, African J Biotechnol 10(37):7225-7235), and the unit content of milbemycin A3+ milbemycin A4 increased to 2237 mg/L in the Chinese patent CN 103468625 A.
The fermentation broth containing milbemycin has complex composition, wherein milbemycin A3 and milbemycin A4 are main components and other structural analogues are further comprised. In general, the ratio of milbemycin A3 to milbemycin A4 in the fermentation broth containing milbemycin is about 1:2, so the existing commercial product is also a mixture of components of A3 and A4. The main reason leading to this result is that it is difficult to separate A3 and A4 single component from the fermentation broth with similar proportions of A3 and A4, which affects the final yield. Achieving the single component production of A4 can further facilitate the application of milbemycin A4 single component in other fields. The most effective way to obtain a single component is to obtain a new strain capable of producing the single component and to further increase the fermentation unit.